This invention relates to techniques for unlocking digital content.
Some publishers publish books and other content on computer readable media such as compact disks (CD's). Other types and formats of digital media may be used to publish a book or other type of printed publication in digital form, such as a digital video disk, magnetic disk and the like. Generally, when content such as books and the like are published in a digital format they are published in a bitmap or PDF type format.
Publishers are understandably concerned about publishing content in digital format, in general, and accessible digital format in particular, because material published in digital format such as on a CD is easily illegally copied and distributed. In digital format, publishers risk losing control of a copyrighted work. Some publishers avoid making some or all of their content available in digital format, or protect the content to make the content difficult to copy and to access. Some schemes use encryption that requires a key for instance to unlock the CD to allow a user to retrieve text representation of the content from the CD.
Accordingly, publishers might want to protect content to prevent users from being able to view the content, duplicate the content exactly (e.g. by duplicating the entire CD), printing the content (e.g. printing a copy of the book from the CD), and extracting content, such as text or images, and using the extracted content by reformatting the extracted content, or translating the extracted content or incorporating the extracted content into some other form, such as for searching or indexing.
PDF format has an elaborate set of protections that address some of these protection issues. For example, in PDF, printing can be locked, as can be copying of text, or passwords can be used to lock and unlock copying and printing. In some applications, PDF uses “back door” methods that allow screen readers for the blind to access text, via secret passwords that publishers share with the manufacturers of the screen readers.